1. Field of the Invention
The present invention relates to a method and apparatus for receiving and/or reproducing digital signals of movies, programs, and the like transmitted via transmission media such as coaxial cables, telephone lines, and broadcast satellites. More particularly, the invention relates to such a method and apparatus capable of constraining a copy of a recorded digital signal.
2. Description of the Related Art
A method of inhibiting a copy of a video signal with a recording/reproducing apparatus is disclosed, for example, in JP-A-61-288582. The technique described in this JP-A-61-288582 adds a signal immediately after a synchronization signal of a video signal so that although a television can display this video signal, a video tape recorder (VTR) can record only an image of poor quality.
Another JP-A-4-360068 discloses techniques of restricting a user to copy data or inhibiting a user to see an image of data, with a data recording/reproducing apparatus.
As a method of compressing digital video signals at a high efficiency, the ITU-T Draft Rec. H. 262 standard called MPEG-2 (Moving Picture Experts Group) is known. MPEG-2 Systems Working Draft is also known which is the transmission standard of video and audio signals compressed by MPEG-2.
The above standards show the techniques of compressing a program and broadcasting it in a digital format. This compression method realizes a large compression rate so that a single transmission channel can broadcast programs four to eight times as many as a conventional analog broadcast. With this technique, services called near video-on-demand are already available in which the same 2-hour movie is repetitively broadcast at an interval of 30 minutes. However, it is impossible to broadcast all programs 24 hours for near video-on-demand. Therefore, subscribers have a great need for recording a program and reproducing it at a desired time to watch it.
In recording/reproducing a digitally compressed and broadcast program, it can be considered that a received digital signal is decompressed and converted into an analog signal to record it with a conventional analog VTR. However, an analog signal recorded in the analog VTR loses a good signal-to-noise (S/N) ratio of digital signals.
It is therefore desired to digitally record a digital broadcast signal. However, no technique is disclosed as yet to record a digital signal compressed and broadcast, for example, in conformity with the MPEG standard. Generally, error correction is performed to a sufficient degree during recording/reproducing digital signals so that even if copies are made repetitively, the image quality is not lowered. However, on the other hand, it is difficult to protect the rights of a copyright holder if a copy without image quality degradation is permitted. Techniques for protecting the rights of a copyright holder are disclosed in the above cited JP-A-61-288582 for conventional analog VTRs. The other JP-A-4-360068 discloses techniques for restricting a user to copy data or inhibiting a user to see an image of data, with a data recording/reproducing apparatus.
However, techniques of recording digital signals compressed and transmitted in conformity with the MPEG standard and restricting copies of recorded digital signals are not shown at all in the above documents.
Although it is desired to directly record a digital broadcast signal itself, the technique of recording digital signals compressed and transmitted in conformity with the MPEG standard is not disclosed as yet. Generally, error correction is performed to a sufficient degree during recording/reproducing digital signals so that it is advantageous in that the same tape can be repetitively viewed without any image quality degradation. However, on the other hand, if the same tape is allowed to be repetitively viewed without any image quality degradation, a video tape like a cell video tape (a commercial video cassette tape of movies or the like) can be formed easily so that protection of the rights of a copyright holder is difficult. JP-A-61-288582 discloses the technique of protecting the rights of a copyright holder for conventional analog VTRs. JP-A-4-360068 discloses the technique of restricting a user to copy data or inhibiting a user to see an image of data, with a data recording/reproducing apparatus.
However, techniques of recording digital signals compressed and transmitted in conformity with the MPEG standard and restricting copies of recorded digital signals are not shown at all.